1. Field of the Invention
The invention relates to a method and apparatus for removing ammonia from wastewaters such as coke plant or coal conversion wastewaters and particularly raw ammonia liquors.
2. Description of Prior Art
Industrial effluents such as coke plant wastewaters often contain high levels of ammonia as well as phenols, cyanides, sulfides, thiocyanates, thiosulfates, and other compounds. In view of the fact that current federal regulations limit the amounts of ammonia and other compounds which may be discharged into bodies of water, numerous attempts at reducing pollutant content have been made.
With respect to coke plant wastewaters, ammonia removal has normally been achieved by means of a steam distillation process operated in conjunction with either lime or caustic addition.
Examples of previous treatment systems may, for example, be found in U.S. Pat. No. 3,278,423 to Millar which discloses a process for the treatment of aqueous crude effluent liquors from coal carbonizing plants. The process which comprises distilling the crude industrial liquor to first remove free ammonia. The partially treated effluent is next subjected to a biochemical oxidation step and is then treated by the addition of lime to convert fixed ammonia salts into free ammonia salts which may then be separated from the waste liquor by means of distillation.
While the addition of lime has indeed proven useful in converting fixed ammonia into free ammonia which may be liberated by means of distillation, such addition has resulted in fouling problems which have proven troublesome when the process is operated on an industrial scale. Thus, several attempts have been made to overcome fouling problems by means of the addition of scale inhibiting compounds.
U.S. Pat. No. 4,104,131 to Didycz et al. discloses a water purification process for the removal of ammonia from wastewaters such as coke plant or coal conversion wastewaters which comprises the addition of lime in amounts sufficient to react with fixed ammonia salts present in the wastewater. The patent discloses the addition of a scale inhibitor compound selected from the class of certain organic phosphonates. The process is conducted in essentially two steps. In the first step, the free ammonia is distilled out of the effluent. The fixed ammonia level is substantially reduced in a second distillation step performed on the wastewater after it has been treated with lime and scale inhibitor compounds.
U.S. Pat. No. 4,108,735 to Burcaw et al traces the history of the use of steam stills to remove ammonia from weak ammonia liquor generated during the coking of coal. In the conventional process, the liquor is first steam distilled in what is called a "free leg" to remove free ammonia and ammonia compounds. The once distilled weak ammonia liquor is then combined with an aqueous slurry of lime and then distilled in what is known as the "fixed" leg of the process where a direct counter current flow of steam distills the hydrated ammonia from the lime-liquor slurry. Both the free and fixed legs of the ammonia still comprise an upright column having internally disposed horizontal plates or trays. Each plate is equipped with gas-liquid contacting means through which ascending steam may pass. The gas-liquid contacting means are conventionally either trays comprising sieve holes or bubble cap assemblies. As the patent points out, a major drawback inherent in using a conventional ammonia still is the tendency of the distillation column to become plugged or fouled. This results because solid calcium compounds which do not dissolve but instead form a dispersion tend to accumulate in and around the gas-liquid contacting means thereby restricting and eventually interrupting or interfering with the flow of steam. Fouling is particularly a problem in the free leg of the still. Once fouling occurs the still must be dismantled and cleaned thus resulting in substantial down time and increased expense. Even prior to complete fouling of the system, by virtue of buildup within the still, the still necessarily operates at reduced efficiency. As a result, ammonia stills for the distillation of weak ammonia liquor derived from coal coking operations have traditionally been designed with an excess capacity so as to be able to withstand a certain degree of fouling.
The patent seeks to overcome the clogging problem normally encountered by subjecting the incoming weak ammonia liquor to a preliminary precipitation with calcium hydroxide prior to its entrance into the ammonia still. Thus, prior to entering the free leg of the system, the liquor is treated in a lime leg which results in the precipitation of insoluble calcium salts prior to distillation. Such pre-precipitation essentially reduces the fluoride and carbonate ion concentrations in the weak ammonia liquor to the point where their calcium salts will not precipitate during distillation and cause clogging. The patent discloses an alternative attempt at overcoming the clogging problem in referring to the addition of caustic soda prior to distillation. By virtue of this substitution, calcium ions which are normally associated with the clogging problem are eliminated from the still. Although such a substitution has been found satisfactory from the point of view of fouling, the cost of caustic soda may vary and can prove very expensive depending on market conditions.
Yet another problem inherent in conventional lime treatment processes is that lime, when added to the effluent, results in a colloidal solution. The solids formed when the lime contacts the waste liquor, make it virtually impossible to automatically control the process by means of sensors, automatic instrumentation, feedback control and the like, by virtue of the fact that the solids make accurate measurement of variables in the effluent impossible.
Furthermore, pH control is very difficult with lime systems since, once having been passed through the free leg of the system, the free ammonia is removed and the system is slightly acidic. Once alkali is added to the liquor, the pH jumps. In view of this swing in pH in the lime system, accurate feedback control of lime addition is extremely difficult.
Finally, although prior treatment processes have referred to the use of biological treatment steps, such treatments have been difficult to achieve in practice and have necessarily entailed extremely careful monitoring of the biological mass and has thus been very expensive. Furthermore, the use of calcium ions in the treated wastewater has severely hindered the addition of phosphates to the wastewater, such phosphates being necessary for an effective biological treatment. When calcium ions are present they will precipitate as calcium phosphate.